Function of the sarcoplasmic reticulum (SR) in hypophosphatemic myopathy

Lipid composition and Ca2+ transport properties were examined in the isolated sarcoplasmic reticulum of non-exercised muscles of markedly phosphorus depleted rats. Phosphorous depletion with a fall of plasma Pi from 8.04 +/- 0.85 to 2.56 +/- 0.48 mg/dl was accompanied by a decrease in muscle Pi and ATP content and by a significant decrease in the phospholipid/protein ratio of sarcoplasmic reticulum. The phosphatidyl -choline/phosphatidyl-ethanolamine ration in sarcoplasmic membranes was increased. Storing capacity for Ca2+ was significantly diminished. In contrast, there was no significant change of kinetic parameters of Ca2+ transport, i.e. of the initial rate of uptake and concentrating ability. These findings do not necessarily exclude changes of cytosolic Ca2+ concentration in phosphate depletion, but they exclude alterations of intrinsic kinetic properties of the sarcoplasmic reticulum as a cause of any such potential changes.